See the screenshot. In theory, once the observer detects a change, the white concrete block will get powered by the repeater, powering the nearby red stone dust, which then powers the sticky piston underneath, the concrete, as well as the red stone lamp. 

What actually happens is the piston does extend, but the lamp (or the dust connecting to it) does not get powered, which means piston is getting (speculatively?) powered with the red stone dust that didn't get powered. 

I've put the other sticky piston there to make sure the piston is not getting powered by anything else, so only the unpowered red stone dust is able to power the piston.

Attaching the world as well, just put something in front of the observer.

I did look around a bit, didn't see a bug that is a duplicate. MCPE-15793 is a common one, but it doesn't seem to match the issue here. I'm sorry if I missed another one that describes this issue, in my defense, there are a lot of piston red stone bugs.